1. Field of the Invention
The present invention relates to a spread spectrum frequency-hopping, multilevel frequency shift keying (FH-MFSK) decoder and, more particularly, to a spread spectrum FH-MFSK decoder for use in a radio receiver where the decoder is capable of directly demodulating a Q level, L-length FH-MFSK radio-frequency input signal into a baseband signal, wherein a desired user's message signal is a sequence of tone bursts at a fixed frequency over an L-length sequence, prior to steps of spectrum analysis and detection of the correct level within the baseband signal corresponding to the desired user's message signal.
2. Description of the Prior Art
Spread spectrum radio communication systems using various modulation techniques have evolved to provide protection of transmitted radio signals from detection, demodulation and/or interference by outside sources. In multiple access spread spectrum communication, each user has access to the whole system bandwidth. One way of distinguishing the signals from different users is to give each user an address consisting of a fixed pattern in time and frequency. The information to be transmitted is modulated or coded onto the address. The receiver detects the appropriate address and decodes the message. This technique is often referred to as random-access-discrete-address (RADA) or code-division multiple access (CDMA).
The two major impairments of mobile radio communication systems are interference from other users and multipath fading. The conventional Frequency Division Multiple Access (FDMA) technique uses guard bands between frequency channels to minimize interference and increased signal power to combat fading. Recently, various frequency-hopping (FH) techniques have been provided for use in mobile and satellite radio systems.
One such technique is disclosed in the article "A Spread Spectrum Technique for High Capacity Mobile Communications" by G. R. Cooper et al in the Conference Record of the Twenty-Seventh Annual IEEE Vehicular Technology Conference at Orlando, Fla. on Mar. 16-18, 1977 at pages 98-103 which relates to a cellular spread spectrum frequency-hopped, differential phase shift keying (FH-DPSK) mobile communication system. In a typical receiver an array of delay lines and bandpass filters selects the desired address waveform out of the incoming signal. The phases of the various elements of the desired signal are detected relative to the previous word and are then passed through a linear combiner and maximum-likelihood decision circuit for proper processing.
Another technique for use with a satellite communication system is disclosed in the article "A Processing Satellite Transponder for Multiple Access by Low-Rate Mobile Users" by A. J. Viterbi in the Conference Record of the Fourth International Conference on Digital Satellite Communications at Montreal, Canada on Oct. 23-25, 1978 at pages 166-174 which relates to a frequency-hopping, multilevel frequency shift-keyed (FH-MFSK) arrangement. In the disclosed receiving section, a set of received 2.sup.K level, L-length signals are spectrally decoded and processed to select a produced L-length transmission as the decoded signal. If two or more L-length signals are produced, resolution would be to arbitrarily choose one as the correct message.
Alternatively, a mobile radio communication system is disclosed in the article "Frequency Hopped Multilevel FSK for Mobile Radio" by D. J. Goodman et al in the Conference Record of the 1980 International Zurich Seminar on Digital Communications Mar. 4-6, 1980, Zurich, Switzerland at pages A5.1-Ab.6 at FIG. 2 thereof, a receiver is shown wherein a signal comprising multiple user's transmitted signals are spectrum analyzed to determine which frequencies were received during each period of a sequence of frequency-hopped signals and a particular user's FH address sequence is subtracted modulo-2.sup.K from such spectral determinations for deriving the decoded signals. A majority logic decision rule is applied to arrive at a correct message signal when noise or multipath propagation influence detection matrix entries.
The prior art frequency-hopping receivers, however, generally spectrum analyze the received signals on a chip-by-chip basis over the L-length sequence and similarly process the resultant analyzed signals to determine the particular user's correct message signal. The problem remaining in the prior art is to provide a decoding arrangement which concurrently demodulates and decodes an rf input signal and also enables the avoidance of a chip-by-chip spectral analysis of a FH-MFSK received signal and thereby permits a simpler receiver arrangement.